This invention relates generally to closed loop air/fuel ratio control of an internal combustion engine mounted on a motor vehicle or the like, and more particularly, the present invention relates to a method and apparatus for controlling the air/fuel ratio in response to a signal derived from an exhaust gas sensor to reduce the emission of noxious components in burnt gases.
Various methods and systems for effecting air/fuel ratio control are known, and in one conventional method, an integration corrective setting or correction factor is derived by integrating the output signal from the gas sensor, and then another corrective setting or correction factor is derived in accordance with the integration correction factor and the operating condition of the engine. The second-mentioned correction factor, which will be referred to as an engine condition correction factor or amount, is stored in a memory so that feedback control will be effected by determining the air/fuel ratio supplied to the engine by correcting or modifying a basic fuel flow amount, which is derived on the basis of the intake airflow and the engine speed, by these correction factors. In such a known system, in which so called learning control or correction is effected, the engine condition correction factor is apt to assume a value far deviated from its standard value due to some reasons. For instance, in an automotive engine, fuel vapor adsorbing and supplying system including a canister is provided to supply the engine with fuel vapor evaporated in the fuel tank. Because of such a system, a rich mixture is apt to be fed to the engine irrespective of the learning control, causing the engine condition correction factor or factors to assume undesirable values which are far deviated from their standard values.
If the engine is stopped under such condition, the data for the engine condition correction factors remain in the memory. Therefore, when the engine is restarted after being cooled, the engine condition correction factors, whose values are far deviated from their standard values, will be used to erroneously control the air/fuel ratio resulting in undesirable operation of the engine and emission of noxious components.